Catalysts are employed in the exhaust system of diesel vehicles to oxidize carbon monoxide (CO) and hydrocarbons (HC), including the volatile organic fraction of particulates, produced during engine operation into carbon dioxide (CO.sub.2). Current diesel after-treatment technologies are being developed to convert nitrogen oxides (NOx), in addition to the other emissions, into more desirable gases. These technologies involve lean NOx catalysts, selective catalytic reduction (SCR) catalysts, and lean NOx traps.
Lean NOx catalysts are catalysts that convert NOx in a lean, i.e., O.sub.2 -rich, environment with the aid of low levels of hydrocarbons. In the case of diesel, hydrocarbon emissions are too low to achieve significant lean NOx conversion, so hydrocarbons need to be added by injection of diesel fuel into the pre-catalyst exhaust stream. Diesel lean NOx catalysts generally include such materials as e.g., precious metals or base metal zeolites. In particular, platinum is used because of its lean NOx activity at low temperatures, i.e., usually less than 230.degree. C. This low temperature activity is needed, e.g., during European urban-type driving where diesel exhaust gas temperatures typically measure between 100-300.degree. C. at the engine exhaust manifold. Overall diesel catalysts need to be able to operate over a wide temperature range, e.g., up to 500.degree. C. Base metal zeolites and precious metal other than Pt have lean NOx activity at higher temperatures, usually greater than 230.degree. C. Precious metal and base metal zeolite materials are deficient, however, because they have a limited temperature range of lean NOx activity and a limited level of NOx conversion over their active temperature range. Platinum has an additional shortcoming of reducing NOx predominantly to N.sub.2 O rather than N.sub.2.
Selective reduction catalysts (SCR), in contrast to using hydrocarbons for NOx conversion, use urea or ammonia to provide NOx conversion in O.sub.2 -rich exhaust. Base metal zeolite materials are used for SCR catalysts on diesel vehicle. SCR catalysts provide much higher NOx conversion than lean NOx catalysts. However, similar to lean NOx catalysts, SCR catalysts are deficient because they have a limited temperature range of operation. Their NOx conversion activity is usually confined to temperatures greater than 230.degree. C.
NOx traps operate on lean-burn gasoline vehicles by absorbing NOx on a material like barium oxide during lean-burn operation, i.e., engine air/fuel (A/F) ratio is ca. 20/1 and exhaust is O.sub.2 -rich. Then, the NOx trap is subjected to engine exhaust during stoichiometric or fuel-rich operation, i.e., A/F ratio is 14.7 or lower and exhaust becomes O.sub.2 deficient. This is done to desorb the NOx and convert it over precious metal in the trap formulation. In diesel systems, the exhaust gases generated by the engine are always oxidizing, i.e., lean, the engine A/F ratio being generally from 20/1 to 60/1. Using a NOx trap like barium oxide on alumina described above is unlikely with a diesel engine. This is because it is impractical to run the engine fuel-rich or near stoichiometric to release and reduce NOx. Hence, using such NOx traps in gasoline lean-burn engine exhaust systems is more commercially appropriate since the air/fuel ratio can more conveniently be made stoichiometric or fuel-rich.
The present invention method overcomes deficiencies of prior diesel exhaust purification methods and provides a catalyst system capable of efficiently reducing nitrogen oxides as well as oxidizing hydrocarbons and carbon monoxide in the relatively cool oxidizing conditions of diesel exhaust gases.